This application claims priority from U.S. Provisional Patent Application Ser. No. 60/533,510, filed Jan. 2, 2004 and from PCT Application PCT/US2004/042778, filed Dec. 17, 2004. The invention relates to vapor generators (boilers) of subcritical pressure, which may serve as heat recovery steam generators (HRSG) providing enhanced cycling capacity and high base-load efficiency for combined cycle or cogeneration power plants. These vapor generators also may find application in other environments, such as regular boilers, boilers used in coal gasification power plants, and so forth. In the HRSG situation, the vapor generators may utilize the heat content of the power plant gas turbine exhaust and/or combustion process for generating live steam and may employ this steam either as a motive fluid for a steam turbine or for process purposes. Presently, the most widely used HRSGs are those with a horizontal mode of construction and vertical heated tubes.
The principles of the present invention will be described mostly by reference to this type of HRSG for the sake of convenience; however, this is not intended to limit the scope of the claims.
Two basic types of steam generators are known, those with and without a separating drum. Drum-type boilers operate either by natural circulation without circulation pumps, or by forced circulation, usually involving circulation pumps. In the case of natural or forced circulation in a drum-type boiler, the circulating water supplied to the boiler evaporator is only partly evaporated when passing through its heated tubes. After separation of the generated steam in the steam drum, the water that is not evaporated is fed again, or recirculated, to the same heated tubes for further evaporation. Drum-type boilers are applicable only for subcritical pressure (<220 bara).
Boilers without separation drums are applicable both for sub- and supercritical pressures. They utilize the once-through concept. Evaporation and steam superheat of the working fluid takes place in the boiler heated tubes without intermediate steam separation and water recirculation. These boilers have the advantage that they do not require a heavy and expensive drum, making them more suitable for fast start-ups and cycling operation with many load changes. However, once-through boilers are not widely used in the HRSG industry, due to their many disadvantages, which are described below.
The traditional drum-type boilers put severe restrictions on the ability to start up the gas turbines quickly; in a typical installation, while the gas turbine itself could start up in approximately twenty minutes, the time required to gradually heat up the drum of the boiler is up to two hours, which delays the full load operation of the gas turbine. This results in a loss of the opportunity to supply power to the grid at times of peak demand, when energy prices are high. Start-up is also a time when the rate of emissions is up to ten times the level during normal operation, and the efficiency of gas turbine power generation is substantially reduced. Thus, there is a market demand for an efficient heat recovery steam generator (HRSG) that would allow quick gas turbine start-up and load ramp rate. One proposed solution is a once-through boiler with a horizontal mode of construction and vertical heated tubes, as described in U.S. Pat. No. 6,189,491 entitled “Steam Generator”. According to this patent, the heating surface of the boiler is “configured in such a way that the fluid circulating in a tube heated to a greater temperature than the following tube of the same continuous heating surface has a higher flow rate than the fluid circulating in the following tube”.
The benefits of using the once-through concept in boiler evaporators with vertical heated tubes have been generally understood for some time. A comprehensive consideration of the thermal and hydraulic principles incorporated in the above mentioned U.S. Pat. No. 6,189,491 can be found in “Steam Boilers of thermal Power Stations” by U. S. Reznikov and Yu. M. Lipov (Mir Publishers, Moscow, 1985, English Translation, pages 135 & 136, FIG. 11.18). This textbook and U.S. Pat. No. 6,189,491 both describe once-through steam generators comprised of analogous components: the entry and discharge collectors; a heating-gas duct; a once-through heating area disposed in the heating-gas duct; the once-through heating area formed from a number of vertically disposed steam-generator tubes connected in parallel between the collectors for a through flow of a working fluid.
It should be noted that the mechanism of interaction between the heat transfer and the features of circulation disclosed in U.S. Pat. No. 6,189,491 is also similar to that found in the least heated evaporator multiple row modules of conventional HRSG with vertical heated tubes and natural circulation. Indeed, due to very low working fluid velocities (sometimes, a small fraction of one m/s), the friction pressure losses in the parallel heated tubes of those modules, even at a full boiler load, are especially low compared with the geodetic pressure drop in the vertical tubes. This makes the mechanism of flow distribution between the heated evaporator tubes of multiple row modules very similar to that found in the patented technology. The references to the book by U. S. Reznikov and Yu. M. Lipov, as well as to the least heated tube modules of conventional HRSGs with natural circulation, are incorporated herein for the purposes of describing the background of this application.
Disadvantages of conventional drum boilers: Thick-walled and heavy steam drums of conventional HRSGs significantly limit their cycling capacity and increase the boiler cost. The undisputed advantage of once-through HRSG with vertical heated tubes, as compared to conventional HRSG with vertical tubes and natural circulation, is that it has no steam drum. This adds to the boiler cycling capacity and provides unrestricted start-up and load ramp rate for the gas turbine.
The drum-type boilers of the prior art have another well-known problem. At high subcritical pressure (close to 170-190 bara) and/or when there are restrictions to the length of their heated vertical tubes, there emerge serious difficulties in providing reliable natural circulation based on the traditional design approaches. Under such conditions, the evaporator heated tubes and the piping system become more complicated and expensive. A number of known risks, such as critical heat flux, circulation stagnation, circulation reversal, etc., sometimes prevent the realization of significant and long-appreciated benefits of natural circulation.
Disadvantages of once-through boilers: The once-through boiler described in the '491 patent has several disadvantages.
(1) Due to low working fluid velocities, it requires a larger evaporator heating surface than a conventional boiler with natural circulation.
(2) It employs a complex piping system for the transportation and distribution of the steam/water mixture in-between the evaporator sections.
(3) Its start-up/shut-down procedure and equipment are more complicated than that of conventional HRSG with natural circulation.
(4) Special throttle/choke devices and/or design approaches (for example, the use of evaporator heated tubes with different inside diameters, as claimed in the U.S. Pat. No. 6,189,491) sometimes are needed for working fluid stability, etc.
(5) In addition to information disclosed in the U.S. patent, there is still a need for research and development work (both experimental and theoretical) dedicated to the interconnected subjects of steam/water mixture distribution, intertube flow pulsations and boiler feedwater flow variations. These negative aspects are of practical importance and likely to be encountered in the once-through boiler of the type described in the '491 patent.